1. Field of the Invention
The present invention relates to a method of producing a bleached pulp. More particularly, the present invention relates to a method of producing a bleached pulp in which method a pulp prepared from a lignocellulosic substance is bleached with chlorine dioxide, and the chlorine dioxide bleaching procedure is carried out under a pressure created by a compressed gas, whereby not only the delignification and bleaching effect is improved and the amount of bleaching chemicals necessary for bleaching the pulp to a desired brightness is reduced, but also the amount of organic chlorine-containing compounds contained in a waste water is reduced.
2. Description of the Related Art
To employ a lignocellulosic substance as a paper-forming material for various uses, it is necessary that the lignocellulosic substance be pulped by a chemical pulping procedure, for example, a digesting procedure or by a mechanical pulping procedure using, for example, a refiner; and the resultant pulp be bleached with bleaching chemicals to enhance the brightness of the pulp. For instance, kraft pulp is usually (except for the case where the kraft pulp is used, for specific paper sheets, for example, packing paper sheets which need a high mechanical strength) subjected to an alkali-oxygen delignification procedure, and then is bleached with a bleaching chemical and bleaching additive, for example, chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, hydrogen peroxide and sodium hydroxide, whereby the coloring substances such as lignin contained in the pulp are removed, and the delignified pulp is bleached to provide a bleached kraft pulp suitable for practical use.
When a bleached pulp is produced from an unbleached pulp, it is necessary that the mechanical strength of the pulp fibers is maintained as high as possible, and that the hydrolysis of hydrocarbon substances such as cellulose and hemicellulose substances, from which the pulp fibers are constituted, be minimized, and thus usually a vigorous single step bleaching procedure is avoided, and a multi-stage bleaching procedure, for example, a 3 to 6 stage bleaching procedure in which various bleaching chemicals can be used in combination with various bleaching conditions, is carried out under medium conditions.
In the conventional multi-stage bleaching method, the following procedures are practically employed. Namely, firstly, a pulp is treated with chlorine to chlorinate the lignin contained in the pulp and to give solubility to the lignin, then the chlorinated pulp is dissolve-extracted with an aqueous alkali solution to isolate and remove the lignin from the pulp, and finally, the residual lignin in a small amount in the pulp is decomposed and removed by using a hypochlorite or chlorine dioxide, to obtain a bleached pulp having a high brightness.
Recently, however, the environmental influence of organic chlorine-containing compounds (which will be represented by AOX hereinafter) contained the waste water discharged from a pulp-chlorination step in the pulp-bleaching procedure has become a problem, and thus a new method of not using chlorine for bleaching the pulp has been promoted. Also, when a hypochlorite is used, chloroform is generated during pulp-bleaching and there is a risk of affecting the environment. Therefore, a new pulp-bleaching sequence using no hypochlorite is demanded.
Recently, as substitutes for the chlorine-containing bleaching chemicals such as chlorine and hypochlorite, oxygen-containing bleaching chemicals, for example, ozone, oxygen, hydrogen peroxide and peracids, for example, peracetic acid and persulfonic acid, and salts of the peracids have drawn the pulp-bleaching industry's attention. These oxygen-containing bleaching chemicals except for oxygen and hydrogen peroxide are, however, disadvantageous in that they exhibit a low selectivity to the delignification and thus a high risk of decreasing the mechanical strength of the pulp, the prices thereof are high, and the chemical handling is difficult because there is a risk of explosion. Due to the above-mentioned disadvantages, the utilization of oxygen-containing bleaching chemicals has not yet become wide-spread. Therefore, in the present stage, as a substitute for chlorine and hypochlorite, chlorine dioxide, of which the practical use result is known, is generally employed. Particularly, in practice, the frequence of replacement of chlorine-bleaching with the chlorine dioxide-bleaching is now increasing. However, since the mechanism of the delignification with chlorine dioxide is different from that with chlorine, the chlorine dioxide must be used in an increased amount or bleaching chemicals in other bleaching stages must be used in a large amount, to obtain the same degree of delignification as that with chlorine.
As a pulp-bleaching method in which a first stage is carried out by using chlorine dioxide in place of chlorine, whereby the brightness of the resultant bleached pulp can be maintained constant and the amounts of chlorine dioxide and other bleaching chemicals used can be decreased, a method disclosed, for example, in J. E. Jiang et al., Appita, 45(1), 19 (1992), wherein the degree of delignification in the pulping step is enhanced, as much as possible, to decrease the Kappa number of the unbleached pulp; a method as disclosed in, for example, Japanese Unexamined Patent Publication No. 4-316,690, in which an alkali-oxygen delignification is promoted by using a pre-treatment chemical, for example, nitrous acid, to reduce the Kappa number of the pulp; a method as disclosed in, for example, Japanese Unexamined Patent Publication No. 2-264,087, in which, before a multi-stage bleaching step, a pulp is treated with a xylan-decomposition enzyme; and a method disclosed in, for example, Japanese Unexamined Patent Publication No. 4-316,689, wherein, before a multi-stage bleaching treatment step, a pulp is treated with a lignin-decomposition enzyme, are known.
The methods mentioned above should be further improved. For example, the method in which the delignification degree in the pulping step is enhanced from the conventional degree to decrease the Kappa number of the pulp is usually disadvantageous in a decrease in the pulp yield and in a decrease in the mechanical strength of the pulp fibers.
The method in which the delignification degree of a pulp in an alkali-oxygen delignification stage is enhanced to decrease the Kappa number of the resultant pulp is disadvantageous in that usually there is a risk of a decrease in the mechanical strength of the pulp. Further, the methods wherein, before the bleaching treatment step, the pulp is pre-treated with an enzyme are advantageous in that the reaction conditions are relatively medium and thus the increases in the mechanical strength of the pulp and in the pulp yield are small and disadvantageous in that the reaction velocity is low and thus a long treatment time is necessary.
The inventors of the present invention have provided a pulp-bleaching method in which an alkali-oxygen-delignified pulp is treated with an acid in an oxygen-containing gas under pressure, whereby the amount of the bleaching chemicals used for the bleaching step can be significantly reduced, and have reported that the acid treatment of the alkali-oxygen delignified pulp in the oxygen-containing gas under pressure can cause the amount of chlorine dioxide used in a first stage of the bleaching step to be reduced to 80% of that in the conventional method.
Also, the inventors of the present invention have provided a pulp-bleaching method in which an alkali-oxygen-delignified pulp is treated with an acid, in a nitrogen-containing gas under pressure, whereby the amount of the bleaching chemicals to be used in the bleaching step can be reduced, and have reported that the acid treatment can cause the amount of chlorine dioxide used in the first stage of the bleaching step to be reduced to about 55% of that in the conventional bleaching method. However, in the method in which the acid treatment causes the amount of the bleaching chemicals in the bleaching step to be reduced, an acid treatment step must be added to the bleaching step and thus the procedure for producing a bleached pulp is prolonged.
A method in which the efficiency of the chlorine dioxide-bleaching step is enhanced by improving the chlorine dioxide-bleaching procedure per se is known. Tappi Jurnal, 75(7), 1992, pages 174 to 180 reports that in a C/E/D/E/D sequence bleaching method, when an initial stage of 5 to 15 minutes, if an chlorine dioxide-bleaching step, was carried out at a pH value of 5 to 10 of an aqueous pulp slurry, and then the remaining stage for 2.5 hours or more was carried out at a pH value of 1.7 to 4.4, a bleached pulp having a desired brightness could be obtained in an amount of chlorine dioxide used in the bleaching step less than that in the conventional method. In the bleaching sequence, C represent a chlorine bleaching, E represents an extraction, and D represents a chlorine dioxide bleaching.
This method is disadvantageous in that this method exhibits no effect for a pulp having a Kappa number of 10 or more, and cannot be applied to an initial chlorine dioxide bleaching stage.
1987 Pulping Conference Proceedings, page 487 reports that in later step of a bleaching method, chlorine dioxide is added in two or more separate stages and in each adding stage, an alkali is also added, to enhance the chlorine dioxide bleaching efficiency. This bleaching method is, however, disadvantageous in that the bleaching method per se is complicated and cannot be applied to an initial chlorine dioxide-bleaching step.
Journal of Pulp and Paper Science, Vol. 22, No. 12, Pages 457 to 463 (1996) reports that in the chlorine dioxide bleaching method, the amount of AOX produced in the chlorine dioxide-bleaching procedure may increase in response to the chlorine dioxide bleaching conditions. Therefore, in the improvement of the chlorine dioxide bleaching method, an increase in the AOX amount should be prevented.